Well, there are a lot of things that could be done. There's a lot of room to expand your core game. Here are some ideas off the top of my head, feel free to accept or reject any of it.
- Room for more interesting weapons. Right now all the weapons I saw would fire in straight lines. A weapon may fire in an arc and be able to hit the top of the island. Or weapons that target specific tiles and have slight homing.
- The intro suggests people live on these islands, Having population could be a possible mechanic. Population could effect the cooldown timers, if you have a lot of people things go more quickly. Having a block of your island blown off could lose some population. People could be something that you find in events on the map. People could also act as raiding parties to the other island.
- FTL had lots of story wrapped up in its map. Story events could lead to fights where you have the choice to engage or not, crew, various weapons and items for your ship. More story events give players a reason to replay.
- FTL had a bunch of unlockable ships that changed up the play style. Made he want to keep replaying to unlock them all and explore how they changed things up. Would be a good idea to explore.
- Fuel that limits how much you can explore of the map, and becomes a resource.
You've got a fun core game that's relatively unique, but it's short and relatively simple right now. There's a lot of room for it to grow into something that people would want to keep returning. Have fun with it!